1. Field of the Invention
The present invention concerns a push-pull amplifier with transformational negative feedback for amplification of variable electrical signals
2. Description of the Related Art
Amplifier circuits with bipolar transistors are generally used for amplification of variable electrical signals. Depending on the circuit, the transistors thereby serve as controlled current or voltage sources that, however, are adapted or biased neither on the input side nor on the output side. In order to be able to use such an amplifier circuit in an amplifier. for RF (radio-frequency) signals of a relatively large bandwidth of, for example, 15 MHz to 300 MHz, the biasing must be extended in a suitable manner. Such RF signals are, for example, used in MR (magnetic resonance) imaging systems, in particular in medical imaging (for example nuclear magnetic resonance tomography). The extension of the amplifier circuit can in particular ensue with negative feedback means with which the input impedance and the output impedance of the amplifier can be adapted such that the impedance of the circuit connected at the amplifier input is at the same level as the amplifier input impedance or at a predetermined scaled level, for example. In the ideal, case the negative feedback even determines the amplification, such that the amplifier operates independent of the exemplar scattering of the employed transistors. This provides a more constant amplification factor of the amplifier across the entire bandwidth, whereby non-linear distortions that are in particular fed back to the transistor are kept low.
Such an amplifier in an emitter circuit with transformational negative feedback is described in “Breitbandige Ferrit-Hochfrequenztransformatoren” (J. v. Parpart, Hüthig Verlag 1997, p. 122-126). The transformational negative feedback is thereby effected by means of a directional coupling transformer. The emitter circuit is connected with an npn transistor such that this operates in single-phase A operation. However, a very high quiescent current is required for this for low-distortion amplification
Furthermore, a push-pull amplifier with transformational negative feedback is described in “Breitbandige Ferrit-Hochfrequenztransformatoren” (J. v. Parpart, Hüthig Verlag 1997, p. 130-133). The aforementioned emitter circuits with transformational negative feedback are thereby symmetrically extended by this same emitter circuit with transformational negative feedback. The signal input and the signal output are thereby respectively connected (via a balancing means in the form of a transformer with a one-sided central tap) with both symmetrically-arranged emitter circuits with transformational negative feedback. In comparison to the aforementioned amplifier type, this amplifier type has the advantage that it exhibits a significantly lower distortion given a lower current consumption. However, due to the use of four radio-frequency transformers in the amplifier, the amplifier has a high technical cost. For example, when the amplifier is used as a component of a radio-frequency circuit each radio-frequency transformer entails inherent disadvantages such as, for example, limited bandwidth, high losses and large space requirements, which must be compensated for in the realization of an amplifier.